Tiny piece (4)
I believe the answer is:
atom
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">tiny piece</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for atom that I've seen before include "A mighty physical particle or bomb" , "Small particle of matter" , "Basic unit of a chemical element" , "Elementary particle" , "Tiny bit" .)